Radiation-resistant circuits from mechanical parts

Microscopic images of two “logic gates” made of microscopic mechanical parts and thus designed to resist ionizing radiation that fries conventional silicon electronics. The top gate performs the logic function named “exclusive or” and the gate in the bottom image performs the function “and.” These devices, designed at the University of Utah, are so small that four of them would fit in the cross section of a single human hair. Image: Massood Tabib-Azar, University of Utah

University
of Utah engineers
designed microscopic mechanical devices that withstand intense radiation and
heat, so they can be used in circuits for robots and computers exposed to
radiation in space, damaged nuclear power plants, or nuclear attack.

The researchers showed the devices kept working despite
intense ionizing radiation and heat by dipping them for two hours into the core
of the University
of Utah’s research
reactor. They also built simple circuits with the devices.

Ionizing radiation can quickly fry electronic circuits, so
heavy shielding must be used on robots such as those sent to help contain the
meltdowns at the Fukushima Daiichi nuclear power plant after Japan’s
catastrophic 2011 earthquake and tsunami.

“Robots were sent to control the troubled reactors, and they
ceased to operate after a few hours because their electronics failed,” says
Massood Tabib-Azar, a professor of electrical and computer engineering at the University of Utah and the Utah Science Technology and
Research initiative.

“We have developed a unique technology that keeps on working
in the presence of ionizing radiation to provide computation power for critical
defense infrastructures,” he says. “Our devices also can be used in deep space
applications in the presence of cosmic ionizing radiation, and can help
robotics to control troubled nuclear reactors without degradation.”

The new devices are “logic gates” that perform logical
operations such as “and” or “not” and are a type of device known as MEMS or
microelectromechanical systems. Each gate takes the place of six to 14 switches
made of conventional silicon electronics.

Development of the new logic gates and their use to build
circuits such as adders and multiplexers is reported in a study set for online
publication in Sensors and Actuators.
The research was conducted by Tabib-Azar, University
of Utah electrical engineering
doctoral student Faisal Chowdhury, and computer engineer Daniel Saab at Case Western Reserve
University in Cleveland.

Tabib-Azar says that if he can obtain more research funding,
“then the next stage would be to build a little computer” using the logic gates
and circuits.

The study was funded by the Defense Advanced Research
Projects Agency.

“Its premier goal is to keep us ready,” says Tabib-Azar. “If
there is a nuclear event, we need to be able to have control systems, say for
radars, to be working to protect the nation. There are lots of defense
applications both in peacetime and wartime that require computers that can
operate in the presence of ionizing radiation.”

In April, the Defense Advanced Research Projects Agency
issued a call for the development of robots to deal with stricken nuclear
reactors to reduce human exposure to deadly radiation. In May, NASA said it was
seeking proposals for new shields or materials able to resist radiation in
space. Circuits built with the new devices also could resist intense heat in
engines to monitor performance, Tabib-Azar says.

MEMS: Ability to withstand
radiation overcomes drawbacks

Current radiation-resistant technologies fall into two categories:
conventional complementary silicon-oxide semiconductor electronics shielded
with lead or other metals, and the use of different materials that inherently
resist radiation.

“Electronic materials and devices by their nature require a
semiconducting channel to carry current, and the channel is controlled by
charges,” Tabib-Azar says. Radiation creates current inside the semiconductor
channel, and “that disrupts the ability of the normal circuitry to control the
current, so the signal gets lost.”

He says the MEMS logic gates are not degraded by ionizing
radiation because they lack semiconducting channels. Instead, electrical
charges make electrodes move to touch each other, thus acting like a switch.

MEMS have their drawbacks, which Tabib-Azar believes is why
no one until now has thought to use them for radiation-resistant circuits.
Silicon electronics are 1,000 times faster, much smaller, and more reliable
because they have no moving parts.

But by having one MEMS device act as a logic gate, instead
of using separate MEMS switches, the number of devices needed for a computer is
reduced by a factor of 10 and the reliability and speed increases, Tabib-Azar
says.

Also, “mechanical switches usually require large voltages
for them to turn on,” Tabib-Azar says. “What we have done is come up with a
technique to form very narrow gaps between the bridges in the logic gates, and
that allows us to activate these devices with very small voltages, namely 1.5 V”
versus 10 V or 20 V. Unlike conventional electronics, which get hot during use,
the logic gates leak much less current and run cooler, so they would last
longer if battery-operated.

Design and reactor
testing of the logic gates

Each logic gate measures about 25-by-25 microns, or millionths of a meter,
“so you could put four of these on the cross section of a human hair,” says
Tabib-Azar. Each gate is only a half-micron thick.

The logic gates each have two “bridges,” which look somewhat
like two tiny microscope slides crossing each other to form a tic-tac-toe
pattern, with tungsten electrodes in the center square. Each bridge is made of
a glass-like silicon nitride insulator with polysilicon under it to give
rigidity. The insulator is etched and covered by metallic strips of tungsten
that serve as electrodes.

“When you charge them, they attract each other and they move
and contact each other. Then current flows,” says Tabib-Azar.

He and his colleagues put the logic gates and conventional
silicon switches to the test, showing the logic gates kept working as they were
repeatedly turned on and off under extreme heat and radiation, while the
silicon switches “shorted out in minutes.”

The devices were placed on a hot plate in a vacuum chamber
and heated to 277 F for an hour.

Three times, the researchers lowered the devices for two
hours into the core of the university’s 90-kW TRIGA research reactor, with
wires extending to the control room so the researchers could monitor their
operation. The logic gates did not fail.

The researchers also tested the logic gates outside the
reactor and oven, running them for some two months and more than a billion
cycles without failure. But to be useful, Tabib-Azar wants to improve that
reliability a millionfold.

Two kinds of logic gates

For the study, Tabib-Azar and colleagues built two kinds of logic gate,
each with two inputs (0 or 1) and thus four possible combinations of inputs
(0-0, 0-1, 1-0, 1-1). The input and output are electrical voltages:

An AND gate, which means “and.” If both inputs—A and B—are
true (or worth 1 each), then the output is true (or equal to 1). If input A or
B or both are false (worth 0), then the output is false (or equal to 0).

An XOR gate, which means “exclusive or.” If input A doesn’t
equal B (so A is 0 and B is 1 or A is 1 and B is 0), the output is true (equal
1). If both A and B are either true (1) or false (0), the output is false (0).

“In a sense, you can say these are switches with multiple
outcomes,” rather than just off-on (0-1), says Tabib-Azar. “But instead of
using six [silicon] switches separately, you have one structure that gives you
the same logic functionality.”

“Let’s say you want to decide whether to go to dinner
tonight, and that depends on if the weather is nice, if you feel like it,” he
says. “In order to make that decision, you have a bunch of ‘or’ statements and
a bunch of ‘and’ statements: ‘I’ll go to dinner if the weather is nice and I
feel like it.’ ‘I like to eat Italian or French.’ You put these statements
together and then you can make a decision.”

“To analyze this using silicon computers,” Tabib-Azar says,
“you need a bunch of on-off switches that have to turn on or off in a
particular sequence to give you the output, whether you go to dinner or not. But
just a single one of these [MEMS logic gate] devices can be designed to perform
this computation for you.”